Nicolas Zenker

Effect of thyroid status and cold stress on tyrosine hydroxylase activity in adrenal gland and brown adipose tissue

Abstract A tyrosine hydroxylase activity (THa) of 1.4 nanomoles DOPA formed per hour per mg of protein has been found in brown adipose tissue homogenate; a value of 30.3 nanomoles was found in the corresponding adrenal homogenate. The THa of brown adipose tissue of normal rat increases markedly upon intermittent exposure to cold temperature and is greatly increased in the thyroidectomized rat. In adrenal gland there is, upon intermittent cold stress, an increase in the THa of normal and of thyroidectomized rats, but no increase in the THa of animal receiving triiodothyronine.

Tyrosine hydroxylase in neuroblastoma.

Abstract Tyrosine hydroxylase activity was assayed in two neuroblastoma tissues. It was found that the neuroblastoma tyrosine hydroxylase required DMPH4 as a cofactor and was stimulated by Fe2+. These properties are quite similar to those reported for bovine and pheochromocytoma enzymes. Catechol compounds and other inhibitors, at concentrations of 2.5 × 10−5-5 × 10−4 m , inhibited the neuroblastoma enzyme to the same extent as bovine adrenal tyrosine hydroxylase. This finding suggests that neuroblastoma tyrosine hydroxylase is normally susceptible to feedback inhibition, in contrast to the pheochromocytoma enzyme which has been reported to be less sensitive to inhibition by catechols. Both neuroblastoma and pheochromocytoma are associated with increased synthesis of catecholamines; in the case of pheochromocytoma there are large stores of norepinephrine in the tumor, but catecholamine synthesis still continues at an elevated rate, apparently because of a decreased sensitivity to feedback inhibition of the tyrosine hydroxylase in this tumor. On the other hand, in neuroblastoma the catecholamines appear to exert a “normal” feedback inhibition on tyrosine hydroxylase, and, therefore, we must look elsewhere to explain the high rate of catecholamine synthesis, such as lowering of the intracellular catecholamine levels by rapid metabolism. Further investigation will be necessary to establish the alteration of control mechanisms in neuroblastoma.

Biogenic amine-depleting effects of benzimidazole-5(6)-dl-alanine☆

Abstract Studies in vivo on benzimidazole-5(6)- dl -alanine were carried out by administering the compound in aqueous solutions intraperitoneally to male Wistar rats. Benzimidazole alanine (278 mg/kg) decreased heart norepinephrine (80 per cent), brain norepinephrine (58 per cent) and brain dopamine (70 per cent) within 3 hr and brain serotonin (70 per cent) within 1 hr of administration. Benzimidazole ethylamine (100 mg/kg) produced a 60 per cent decrease in heart norepinephrine within 3 hr of administration and benzimidazole alanine was not able to reduce heart norepinephrine in animals pretreated with the decarboxylase inhibitor, dl -seryl-2,3,4-trihydroxybenzylhydrazine hydrochloride (Ro-4-4602 before benzimidazole alanine administration). Benzimidazole alanine, administered prior to tranylcypromine completely blocked the increase in brain norepinephrine and dopamine after monoamine oxidase inhibition and reduced the increase in brain serotonin by 65 per cent.

N-methylation of benzimidazole by catechol-O-methyltransferase

Abstract The substrate specificity of catechol- O -methyltransferase (COMT) has been extended in the present study to include benzimidazole, a non-catechol compound. Gas chromatography-mass spectrometry. thin-layer chromatography, and reverse isotope dilution analysis were used to verify the identity of the product of this reaction as 1-methylbenzimidazole. This is the first demonstration of an N -methylation by COMT. The conclusion is reached that benzimidazole and catechol are bioisosteric molecules. The implication of this in the pharmacology of adrenergic systems is discussed.

Effects of DL-3-(5-benzimidazolyl)-2methylalanine on brain and heart catecholamines—I. Depleting effects

Abstract The intraperitoneal administration of DL -3-(5-benzimidazolyl)-2-methylalanine (MBA) causes a marked decline in rat brain and heart catecholamines. The release of tritiated norepinephrine from the rat heart by MBA indicates a release component in the action of the compound. The releasing potency of decarboxylated MBA and the inability of MBA to lower heart norepinephrine in the presence of a decarboxylase inhibitor implicate a decarboxylation product of the amino acid as responsible for the release of norepinephrine by MBA. When MBA and then a monoamine oxidase inhibitor are administered to reserpinized rats, the level of brain amines remains constant, indicating inhibition of norepinephrine synthesis in vivo . The activity of tissue tyrosine hydroxylase in MBA-treated rats confirms this inhibition.

Mode of death effect on rat liver iodothyronine 5′ deiodinase activity: Role of adenosine 3′,5′ monophosphate

Liver thyronine 5-deiodinase activity assayed in crude homogenates in the absence of dithiothreitol (DTT) is increased in rats killed by asphyxia when compared to that of animals killed by phenobarbital injection or decapitation. The addition of cyclic adenosine monophosphate leads to a consistent decrease in observed deiodinase activity, suggesting the possible involvement of this nucleotide in the regulation of this enzyme. The addition of DTT eliminates this effect and suggest a dual regulation of the enzyme by cAMP and physiological sulfhydryl compounds.

Isopropyldiiodothyronine and α-methylthyroxine: Comparison of their in vitro and in vivo effects with those of thyroid hormones

Abstract The in vivo and d in vitro effects of the two thyroid hormone analogs 3,5-diiodo-3′-isopropyl- L -thyronine (IPT 2 ) and of α-methyl- DL -thyroxine (MT 4 ) have been compared to those of the thyroid hormones 3,5,3′-triiodo- L -thyronine (T 3 ) and thyroxine (T 4 ). Against purified glutamate, isocitrate and alcohol dehydrogenases IPT 2 and MT 4 exert inhibitory effects which are quite similar to those exerted by T 3 and T 4 . The effects of the analogs on isolated mitochondria follow closely those of the hormones i.e. they uncouple phosphorylations and inhibit electron transfer along the respiratory chain. When thyroidectomized animals are treated with analogs (5 nmoles/100 g/day for 15 days) their effects on two accepted parameters of thyromimetic activity, metabolic rate (BMR) and α-glycerophosphate (GPD) induction, are quite different. After IPT 2 -treatment, the rise in MR and GPD indicates a hyperthyroid state, whereas MT 4 -treatment results in very incomplete compensation of th hypothyroid state. However, treatment of the thyroidectomized animal with IPT 2 or MT 4 has the same effect on hepatic mitochondrial glutamate dehydrogenase as treatment with T 4 . A relation between thyroid state and adrenal tyrosine hydroxylase (TH) activity has been established; TH activity which decreases in the thyroidectomized animal, is brought back to normal by administration of T 4 and above normal by that of IPT 2 .